Self-assembled monolayers of subphthalocyanines on gold substrates

Article abstract of DOI:10.1021/ol100984d

A series of dithiolane-susbstituted subphthalocyanines have been synthesized that can form self-assembled monolayers on gold surfaces, as confirmed by diverse characterization techniques.

Full text of DOI:10.1021/ol100984d

ORGANIC
LETTERS
2010
Vol. 12, No. 13
2970-2973
Self-Assembled Monolayers of
Subphthalocyanines on Gold Substrates
David Gonza´lez-Rodr´ıguez,† M. Victoria Mart´ınez-D´ıaz,† Julia Abel,‡ Andras Perl,§
Jurriaan Huskens,*,§ Luis Echegoyen,*,‡ and Toma´s Torres*,†
Departamento de Qu´ımica Orga´nica, Facultad de Ciencias, UniVersidad Auto´noma de
Madrid, 28049 Madrid, Spain, Department of Chemistry, Clemson UniVersity,
Clemson, South Carolina 29634, and Molecular Nanofabrication group,
MESA+ Institute for Nanotechnology, UniVersity of Twente,
PO Box 217, Enschede, The Netherlands
tomas.torres@uam.es; j.huskens@utwente.nl; luis@clemson.edu
Received April 29, 2010
ABSTRACT
A series of dithiolane-susbstituted subphthalocyanines have been synthesized that can form self-assembled monolayers on gold surfaces, as
confirmed by diverse characterization techniques.
Subphthalocyanines (SubPcs)1 are versatile chromophores
with outstanding, tunable properties for photoinduced electron/
energy transfer,2 optical data storage,3 nonlinear optics,4 or
sensing applications.5 The organization of these aromatic
macrocycles on metal substrates, in the form of nanoclusters
or flat monolayers, has great potential for the development
of different organic materials. Thin films of SubPcs have
been built using several techniques, namely, spin-coating,6
vacuum deposition onto metal substrates,7 or Langmuir-
Blodgett techniques.8 However, none of these techniques
† Universidad Auto´noma de Madrid.
(4) (a) del Rey, B.; Keller, U.; Torres, T.; Rojo, G.; Agullo´-Lo´pez, F.;
Nonell, S.; Marti, C.; Brasselet, S.; Ledoux, I.; Zyss, J. J. Am. Chem. Soc.
1998, 120, 12808. (b) Mart´ın, G.; Rojo, G.; Agullo´-Lo´pez, F.; Ferro, V. R.;
Garc´ıa de la Vega, J. M.; Mart´ınez-D´ıaz, V. M.; Torres, T.; Ledoux, I.;
Zyss, J. J. Phys. Chem. B 2002, 106, 13139. (c) Claessens, C. G.; Gonza´lez-
Rodr´ıguez, D.; Torres, T.; Mart´ın, G.; Agullo´-Lo´pez, F.; Ledoux, I.; Zyss,
J.; Ferro, V. R.; Garc´ıa de la Vega, J. M. J. Phys. Chem. B 2005, 109,
3800.
‡ Clemson University.
§ University of Twente.
(1) (a) Claessens, C. G.; Gonza´lez-Rodr´ıguez, D.; Torres, T. Chem. ReV.
2002, 102, 835. (b) Torres, T. Angew.Chem. Int. Ed. 2006, 45, 2834.
(2) (a) Gonza´lez-Rodr´ıguez, D.; Torres, T.; Guldi, D. M.; Rivera, J.;
Herranz, M. A.; Echegoyen, L. J. Am. Chem. Soc. 2004, 126, 6301. (b)
Gonza´lez-Rodr´ıguez, D.; Claessens, C. G.; Torres, T.; Liu, S.-G.; Echegoyen,
L.; Vila, N.; Nonell, S. Chem.sEur. J. 2005, 11, 3881. (c) Gonza´lez-
Rodr´ıguez, D.; Torres, T.; Olmstead, M. M.; Rivera, J.; Herranz, M. A.;
Echegoyen, L.; Atienza-Castellanos, C.; Guldi, D. M. J. Am. Chem. Soc.
2006, 128, 10680. (d) Medina, A.; Claessens, C. G.; Rahman, G. M. A.;
Lamsabhi, A. M.; Mo, O.; Yan˜ez, M.; Guldi, D. M.; Torres, T. Chem.
Commun. 2008, 1759. (e) Gonza´lez-Rodr´ıguez, D.; Torres, T.; Herranz,
M. A.; Echegoyen, L.; Carbonell, E.; Guldi, D. M. Chem.sEur. J. 2008,
14, 7670. (f) Gonza´lez-Rodr´ıguez, D.; Carbonell, E.; Guldi, D. M.; Torres,
T. Angew.Chem. Int. Ed. 2009, 48, 8032.
(5) (a) Xu, S.; Chen, K.; Tian, K. J. Mater. Chem. 2005, 15, 2676. (b)
Palomares, E.; Mart´ınez-D´ıaz, M. V.; Torres, T.; Coronado, E. AdV. Funct.
Mater. 2006, 16, 1166.
(6) Rojo, G.; Hierro, A.; D´ıaz-Garc´ıa, M. A.; Agullo´-Lo´pez, F.; del Rey,
B.; Sastre, A.; Torres, T. Appl. Phys. Lett. 1997, 70, 1802.
(7) (a) de Wild, M.; Berner, S.; Suzuki, H.; Yanagi, H.; Schlettwein,
D.; Ivan, S.; Baratoff, A.; Güntherodt, H.-J.; Jung, T. A. ChemPhysChem
2002, 10, 881. (b) Yanagi, H.; Ikuta, K.; Mukai, H.; Shibutani, T. Nano
Lett. 2002, 2, 951. (c) Suzuki, H.; Miki, H.; Yokoyama, S.; Mashiko, S. J.
Phys. Chem. B 2003, 107, 3659.
(3) Wang, Y.; Gu, D.; Gan, F. Phys. Status Solidi A 2001, 186, 71.
10.1021/ol100984d  2010 American Chemical Society
Published on Web 06/01/2010

permit an actual chemical attachment of the molecules onto
the surface and hence do not usually provide fine control
over their arrangement and conformation, which is especially
interesting in the case of these cone-shaped macrocycles.
Here we report for the first time the formation of self-
assembled monolayers (SAMs) of 1,2-dithiolane-substituted
SubPcs on gold surfaces and study them by diverse tech-
niques.
between thioctic acid and the free hydroxy group in 3H and
3F. The synthesis of 2, on the other hand, started from
4-iodophthalonitrile, which was subjected to a cyclotrimer-
ization reaction that led to triodoSubPc 7 as a 1:3 mixture
of C3/C1 regioisomers.10 The reaction between 7 and 4-tert-
octylphenol, followed by chromatographic separation of the
1:3 mixture of regioisomers yielded the C3-symmetric SubPc
6. This compound was then subjected to a Suzuki reaction
with (2-hydroxyphenyl)pinacol boronate,11 providing trihy-
droxy derivative 5 in good yields. An acylation reaction
between 5 and 3 equiv of thioctic acid finally led to
compound 2. Detailed synthetic procedures and characteriza-
tion data may be found in Supporting Information.
Prior to deposition on gold electrodes, cyclic voltammetry
(CV) experiments of compounds 1H, 1F, and 2 were
performed in THF and acetonitrile (see Figure S1 and Table
S1 in Supporting Information). The electron-donor/acceptor
nature of the peripheral substituents on the macrocycle has
a clear effect on the oxidation and reduction potentials of
the SubPcs. For instance, the most relevant redox process,
the first reduction of the macrocycle, occurs at ca. -1.5 V
for SubPcs 1H or 2 but at ca. -1.0 V for fluorinated
compound 1F, which also displays a more reversible
character.
Monolayer formation was first monitored by CV. A gold
electrode was immersed into a solution of the SubPc in
acetonitrile or THF, and the mixture was stirred overnight.
Both 2 and 1H SAMs showed cyclic voltammograms with
a first reduction wave around -1.5 V (vs Fc/Fc+) (Figure
2a and Figure S2 in Supporting Information). However, in
contrast with the experiments in solution, the peak to peak
separation was small, indicating that the electroactive species
are confined to the gold electrode, since it does not follow
the typical diffusion behavior. Unexpectedly, no voltammet-
ric response was observed for surface-bound 1F, despite the
structural similarity with SubPc 1H. This is most probably
due to the reductive stripping of the monolayer. In fact, the
SAMs of 1H also exhibit a clear reduction in current intensity
with successive scans (Figure 2a), which is not surprising
given the very negative potential necessary to reduce the
monolayer (it has been claimed12 that the available window
for thiol SAM stability on gold is -0.7 V to +0.5 V vs Fc/
Fc+). Compound 2 exhibited the best SAM stability in
voltammetric measurements, presumably due to the increased
number of S-Au bonds per molecule.
Figure 1. (a) Structure of SubPcs 1H, 1F and 2. (b) Molecular
models of their respective monolayers on a gold surface.
Three different SubPc molecules substituted with a thioctic
ester moiety were designed and prepared in this work (Figure
1a). Depending on the position of the anchoring group (i.e.,
axial or peripheral), two distinct binding topologies to the
substrate were expected. Compounds 1H and 1F, which bear
the thioctic group on the axial position of the macrocycle,
were designed to form monolayers in which the concave side
of the macrocycle points outward from the gold surface. On
the contrary, SubPc 2 is supposed to arrange with the concave
side facing the substrate, providing the three thioctic groups
bind to the surface (see models in Figure 1b).
The synthesis of compounds 1H and 1F was performed
Via a similar route (Scheme 1). SubPcs 4H and 4F were
first prepared by condensation reaction of phthalonitrile of
tetrafluorophthalonitrile, respectively, in the presence of
BCl3.9 The axial chlorine atom in SubPcs 4H and 4F was
then replaced by hydroquinone to yield 3H and 3F. Finally
the thioctic moiety was coupled Via an esterification reaction
We could estimate the surface coverage values (SC) from
the current flow in the first reductive scan of the monolayers
formed from 1H and 2 (Table 1). The values obtained are
high and compatible with a commensurate layer, which was
calculated to be around 7 × 10-10 mol cm-2.13
Impedance measurements were also performed on these
SubPc-based SAMs to have an idea of the insulating effect
(10) Claessens, C. G.; Torres, T. Tetrahedron Lett. 2000, 41, 6361.
(11) Gonza´lez-Rodr´ıguez, D.; Torres, T. Eur. J. Org. Chem. 2009, 1871–
1879.
(8) Mart´ınez-D´ıaz, M. V.; del Rey, B.; Torres, T.; Agricole, B.;
Mingotaud, C.; Cuvillier, N.; Rojo, G.; Agullo´-Lo´pez, F. J. Mater. Chem.
1999, 9, 1521.
(12) Friggeri, A.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir
1998, 14, 5457.
(9) Claessens, C. G.; Gonza´lez-Rodr´ıguez, D.; del Rey, B.; Torres, T.;
Mark, G.; Schuchmann, H.-P.; von Sonntag, C.; MacDonald, J. G.; Nohr,
R. S. Eur. J. Org. Chem. 2003, 2547–2551.
(13) (a) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem.
1991, 310, 335. (b) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.;
Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687.
Org. Lett., Vol. 12, No. 13, 2010
2971

Scheme 1. Synthesis of SubPcs 1H, 1F, and 2
of the monolayer (Figure 2b).14 These measurements were
the SubPcs in acetonitrile overnight. The substrates were then
studied by several techniques, namely, water contact angle
goniometry, X-ray photoelectron spectroscopy (XPS), and
atomic force microscopy (AFM), which confirmed the
functionalization of the gold surface by compounds 1H, 1F
and 2.
carried out at the half wave potential of ferrocene and, in
agreement with the calculated surface coverage values,
showed that the SAMs are efficient blockers. An analysis of
the curve returns an RCT value of ca. 35000 Ω for 2, which
is a very high value for a SAM. Again, SubPc 1F showed
little blocking effect of the redox probe, as shown in Figure
S3, likely due to the stripping mentioned before.
Contact angle goniometry provides information about the
polarity of the SAMs. As shown in Table 1, the high θa and
θr values measured indicate a relatively hydrophobic surface,
which is in accordance with the presence of the nonpolar
molecules covering the surface. On the other hand, the
hysteresis values are very low, which indicates dense
packing.
“Molecular ruler”15 studies were also preformed by AFM
to determine the thickness of the SubPc SAMs prepared by
Table 1. Half-Wave First Reduction Potentials, Calculated
Surface Coverage Values, Advancing and Receding Water
Contact Angles, Percentage of Sulfur Bound to Gold, and
Height of SAMs for compounds 1H, 1F, and 2
Figure 2. (a) Cyclic voltammograms of SAMs formed from
compound 1H on a gold electrode. The effect of repeated cycling
on the current intensity is shown. (b) Impedance experiment for
compound 2.
1H
1F
2
red
E1/2 (solution), V
-1.532
-1.468
5.26 × 10-10
71/57
-1.000
-1.450
-1.411
9.50 × 10-10
73/66
red
E1/2 (SAM), V
SC, mol cm-2
θa/θr, deg
SAMs were also prepared on silicon substrates covered
with a 20 nm gold layer by exposing them to a solution of
63/55
1.2
height, nm
0.8
1.7
Sbound, %
45
(14) Boubour, E.; Lennox, R. B. Langmuir 2000, 16, 4222.
2972
Org. Lett., Vol. 12, No. 13, 2010

immersion in solution. Octadecanethiol (ODT) circles of a
few micrometers in diameter were microcontact printed on
bare gold surfaces that were afterward immersed in clear
acetonitrile (in the case of the blank) or in the SubPc solution,
in order to grow SAMs of 1H, 1F, and 2 on the free areas
of the gold surface. Measuring the AFM height inside and
outside the ODT SAM areas with respect to the blank
provides an estimate of the thickness of the formed SubPc
SAMs. The measured height differences, shown in Table 1,
Figure 3, and Figure S4 in Supporting Information, yield a
sulfur to gold results in a shift to lower energies of the XPS
signal. The weak S2p 3/2 and S2p 1/2 peaks from the sulfur signal
were monitored, and the fitting with two double S2p peaks
was used to derive the approximate fraction of bound sulfur
(see Figure S5 in Supporting Information). The results show
that about half of the sulfur atoms in 2 are actually bound to
the surface. Such a low value seems to point to an alternative
molecular orientation to the one shown in Figure 1b in which
compound 2 may pack standing up sideways, so that the XPS
number is reflecting a mixture of molecules with one or two
thioctic groups bound. This may be a consequence of a
preferred conformation of the molecules on the surface in
order to maximize the coverage and/or minimize conforma-
tional strain.
To conclude, the different techniques employed in the
characterization of the SubPc SAMs suggest that all com-
pounds form stable, densely packed monolayers on gold
substrates. The AFM height analyses are consistent with the
expected dimensions of a monolayer of SubPcs 1H, 1F, and
2 on the gold surface. Future investigations on SubPc-based
SAMs will be directed to increase their electrochemical
stability and to exploit their supramolecular potential in the
recognition of shape-complementary molecules such as, for
example, fullerenes.2f,17
Acknowledgment. This work has been supported by the
Spanish MEC (CTQ-2008-00418/BQU and CONSOLIDER
INGENIO 2010, CSD2007-00010), the Comunidad de
Madrid (MADRISOLAR-2, S2009/PPQ/1533), the ESF-
MEC (MAT2006-28180-E, SOHYDS). D.G.R. would like
to acknowledge a Marie Curie Reintegration Grant (230964).
L.E. is grateful for financial support from the NSF, grant
DMR-0809129.
Figure 3. Height cross sections and contact-mode AFM height
images of a gold surface patterned with ODT and immersed into
(a) pure acetonitrile, and (b) a SubPc 1H solution in acetonitrile
(0.035 mg/mL). Any value is an average of the points on a cross-
line within the marked area.
Supporting Information Available: General methods,
synthetic procedures and characterization data for all new
SubPc products. This material is available free of charge via
the Internet at http://pubs.acs.org.
OL100984D
monolayer thickness of 2.1 nm for ODT, which is consistent
with the literature value, ca. 1 nm for 1H and 1F, and 1.7
nm for 2, which is in good agreement with the expected
dimensions of the bound SubPcs considering the flexible
nature of the thioctic linker.
(15) Levi, S. A.; Guatteri, P.; van Veggel, F. C. J. M.; Vangso, G. J.;
Dalcanale, E.; Reinhoudt, D. N. Angew.Chem. Int. Ed. 2001, 40, 1892.
(16) Castner, D. G.; Hindsa, K.; Grainger, D. W. Langmuir 1996, 12,
5083.
(17) (a) Claessens, C. G.; Torres, T. Chem. Commun. 2004, 1298. (b)
Claessens, C. G.; Gonza´lez-Rodr´ıguez, D.; Iglesias, R. S.; Torres, T. C. R.
Chim. 2006, 9, 1094.
XPS was employed to determine the average ratio of
bound to unbound sulfur in the SAMs of 2.16 Adsorption of
Org. Lett., Vol. 12, No. 13, 2010
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